板料成形极限理论与实验研究进展

成形极限是板材成形领域中重要的性能指标和工艺参数。文章在阐述成形极限在板料成形中的意义的基础上,综述并分析了成形极限在理论和实验方面的研究进展。成形极限图受应变路径的影响,给工业生产应用带来极大不便。以极限应力构成的成形极限应力图不受应变路径的影响,作为复杂加载路径的成形极限判据更加方便和实用。FLSD研究与FLD相结合,成为精确地确定破裂判别准则的主要途径之一,是近来研究的热点。十字形双向拉伸是实现复杂加载路径有效实用的试验方法。最后对成形极限应力图和十字形双向拉伸试验需要解决的关键问题作了阐述。     

Logistic regression analysis for experimental determination of forming limit diagrams

The forming limit diagram (FLD) is probably the most common representation of sheet metal formability and can be defined as the locus of the principal planar strains where failure is most likely to occur. Experimental determination of the FLD consists in performing a set of formability tests on a sheet metal blank, where a regular grid has been previously etched. After each test, the deformation of the grid is measured and the relative strains computed. Strains observed closely at the fracture location are related to as ‘failed’ points, while strains observed on the sound areas of the specimens are labelled as ‘safe’ points. Starting from a set of experimental tests, the FLD should be empirically determined through a statistical analysis of collected data. In fact, statistical approaches (such as linear regression) are required to properly account for the internal randomness of failure occurrence. Linear regression, as well as most of the other empirical approaches in the scientific literature, takes into account only information related to the safe points.This paper proposes a different approach, the logistic regression, for the empirical determination of FLDs. Logistic regression allows to directly derive the probability of an event (e.g. the failure) as a function of different predictor variables (both the principal planar strains). Therefore, by using logistic regression, the process designer can directly associate the failure probability to the scrapping costs, in order to economically evaluate a new sheet metal forming operation.Logistic regression allows the determination of the FLD by including information concerning both safe and failed points.     

Circle grid analysis applied to the production problems of the car body panel

Production problems of the autobody door inner panel in the press shop is investigated with aid of the circle grid analysis. The aim is to reduce the scrap rate. The strain distribution and the strain history in the severe deformed area is determined. Strains are compared with the limit strains in the form of the forming limit diagrams. Two forming limit diagrams (FLDs) are determined for two sheet metal blank orientations to the rolling direction =90° and =0°, respectively. Mathematical statistics is used for evaluating of experimental data. Statistically significant difference between forming limit curves is appreciated and the formability stock in the range of various strain states through FLD is determined. Also strain history in the investigated area of the stamped part is determined.     

Analysis of plastic flow localization under strain paths changes and its coupling with finite element simulation in sheet metal forming

Formability of sheet metal is usually assessed by the useful concept of forming limit diagrams (FLD) and forming limit curves (FLC) represent a first safety criterion for deep drawing operations. The level of FLC is strongly strain path dependent as observed by experimental and numerical results and therefore non-proportional strain paths need to be incorporated when analyzing formability of sheet metal components. Simulations using finite element method allow accurate predictions of stress and strain distributions in complex stamped parts. However, the prediction of localized necking is a difficult task and the combination of forming limit diagram analysis with finite element simulations often fail to give the right answer, if complex strain paths are not included in these predictions.     

Formability limits by fracture in sheet metal forming

The aim of this paper is twofold: first, to revisit the forming limit diagram (FLD) in the light of fundamental concepts of plasticity, damage and ductile fracture mechanics and, second, to propose a new experimental methodology to determine the formability limits by fracture in sheet metal forming. The first objective makes use of the theory of plasticity applied to proportional strain loading paths, under plane stress conditions, to analyze the fracture forming limit line (FFL) and to introduce the shear fracture forming limit line (SFFL). The second objective makes use of single point incremental forming (SPIF), torsion and plane shear tests to determine the experimental values of the in-plane strains at the onset of fracture. Results show that the proposed methodology provides an easy and efficient procedure to characterize the formability limits by fracture in sheet metal forming. In particular, the paper shows that the FFL determined by means of tensile and conventional sheet formability tests is identical to that determined from SPIF tests on conical and pyramidal truncated specimens. The new proposed approach is expected to have impact in the established methodologies to outline the formability limits on the basis of the forming limit curves (FLC's) at the onset of necking.     

变压边力对矩形件成形性能的影响

起皱和断裂是板料成形过程的主要失效模式 ,合理控制成形过程中压边力 ,可以消除这些缺陷 ,提高成形性能。本文通过对随位置变化的变压边力作用下的矩形盒拉深过程进行数值模拟 ,研究各部位压边力变化对整体成形性能影响、及其影响范围 ,为分块压边圈的压边力的调整提供一定的依据。     

多点位控制压边数值模拟研究

板料冲压成形的主要缺陷是起皱和撕裂 ,当模具的几何形状确定后 ,压边力是关键的成形控制参数 ,它不仅是凸模行程或时间的函数 ,还应随压边位置的不同而变化。确定拉深过程中板料各部位所需压边力的变化规律 ,可实现压边力的优化控制 ,对复杂拉深件如汽车覆盖件的成形尤为有利。该文以非轴对称抛物面车灯反光罩为例 ,采用DYNAFORM软件对其成形过程进行了数值模拟 ,比较了均布控制压边力和多点位控制压边力对成形的影响     

Effect of punch speed on the formability behavior of austenitic stainless steel type 304L

The aim of the present study is to investigate the effect of punch speed on forming limit diagram (FLD) and the formability of austenitic stainless steel type 304L. Effect of strain rate on the height of dome is studied using the hemispherical punch test. Results of this study show that strain rate has significant effect on FLD in this material and high formability obtains at low strain rate. The safe area of FLD between major and minor strains is extended under low strain rate. It is seen that at low punch speed, failure and fracture occur at the pole region (top of the dome), whereas at higher forming rates, failure occurs close to the flange region. Modeling studies are also carried out using Ls-Dyna to know the region of high stress concentration and to predict the location of fracture. There is good agreement between simulation and experimental results.     

Time dependent determination of forming limit diagrams

The forming limit diagram (FLD) is a convenient tool for classification of sheet metals’ formability in the finite element analysis as well as in the press shop. The FLD indicates the maximum strain values which can be applied on a material without failure as a function of the strain condition. In contrast to the standardized evaluation method described in the standard ISO 12004-2 a new time dependent analysis method is presented. Using a regression analysis the onset of necking can be detected automatically independent of the strain state. Results will be presented and discussed in contrast to the existing standard procedure.     

Forming Limit Diagram Prediction of Tailor-Welded Blank Using Experimental and Numerical Methods

The forming limit diagram (FLD) is a useful method for characterizing the formability of sheet metals. In this article, different numerical models were used to investigate the FLD of tailor-welded blank (TWB). TWBs were CO₂ laser-welded samples of interstitial-free (IF) steel sheets with difference in thickness. The results of the numerical models were compared with the experimental FLD as well as with the empirical model proposed by the North American Deep Drawing Research Group. The emphasis of this investigation is to determine the performance of these different approaches in predicting the FLD. These numerical models for FLD are: second derivative of thinning (SDT), effective strain rate (ESR), major strain rate (MSR), thickness strain rate (TSR), and thickness gradient (TG). Results of this research show necking will be happened, when the value of MSR, TSR, ESR criteria is maximum, TG????0.78 and SDT criterion has the first peak in forming process time. The value of dome height of TWB samples at failure was predicted based on the numerical models for samples with different widths. These numerical predictions were compared with the experimental results. The SDT model indicates a better agreement with experimental results in prediction of both the FLD and the limit dome height (LDH) in comparison to the other numerical models. Both numerical and experimental results show that minimum of LDH is happened in plane strain condition.     

7075-T6铝合金板温热成形极限图实验

研究7075-T6铝合金板在温热状态下成形性能,采用电化学腐蚀网格法,利用热力耦合条件下的通用板材成形性能实验机和网格应变自动测量分析系统,获得了7075-T6铝合金板在温热状态下(室温~200℃)的成形极限图(FLD)。实验表明,7075-T6铝合金板的成形极限曲线受温度影响显著,并随温度的升高而上升。基于实验数据,建立了不同温度下7075-T6铝合金板成形极限图的计算模型。     

汽车大型铝合金覆盖件的充液成形技术

铝合金覆盖件的应用是汽车轻量化的关键,但其制造难度较大。通过研究铝合金发动机罩外板的充液成形工艺过程,了解先进柔性技术在汽车领域应用的可行性。分析了工艺参数中液室压力与凸模行程匹配及压边力对板料减薄率的影响,从成形极限上判断零件无起皱、破裂现象的范围。获得最优的整形液室压力为12～20 MPa,压边力过小,板料不能充分塑性变形,压边力过大,板料易失稳破裂,最优的恒定压边力范围为1600～2000 k N。研究表明,采用板材充液成形柔性制造工艺,可降低噪音,无冲击线,且由于液室压力的作用,滑移线减小,可提高大型铝合金弱刚度板材的质量。     

镁合金板材的固体颗粒介质拉深工艺参数

提出基于固体颗粒介质成形(SGMF)工艺的镁合金板材差温拉深工艺,并展开试验研究。通过对AZ31B镁合金薄板进行差温拉深成形试验,研究了成形温度、拉深速度、压边力、压边间隙、凹模圆角和润滑条件对拉深性能的影响,确定AZ31B镁合金板料最佳成形工艺参数。结果表明:该工艺可显著提高镁合金板材的成形性能,成形温度及拉深速度对板料拉深性能影响较大,板料最佳成形温度区间为290～310℃,颗粒介质与板料理想温差为110～150℃;压边力和压边间隙对拉深性能产生联合影响;此外,凹模圆角和润滑条件也对拉深性能有一定的影响。当上述工艺参数达到最佳值时成功拉深出极限拉深比(LDR)为2.41的工件。     

Formability of a more randomly textured magnesium alloy sheet: Application of an improved warm sheet formability test

The warm formability of three sheet magnesium alloys was measured using the OSU formability test adapted for testing at elevated temperatures under isothermal conditions. The adapted test is shown to reliably enforce plane strain tension over a significant fraction of the sample, thus providing an assessment of FLD(0), typically the minimum major strain value on a forming limit diagram. By mathematically modeling the strain as a function of punch displacement, a case is made that the punch displacement itself provides an expedient approach to ranking the relative formability of sheet metals. Combined with knowledge of the constitutive behavior of the material, the punch displacement–strain relationship provides an explanation for the observed shape of the punch load versus displacement curves. OSU formability test results show that a new magnesium sheet alloy, yttrium-containing ZW41, is significantly more formable than traditional magnesium alloys AZ31 and ZK10. The improvement is linked to a more random texture in the new alloy, which diminishes the tendency for gross, catastrophic shear instability typical of the strongly textured traditional alloys.     

A New Methodology for Ductile Fracture Criteria to Predict the Forming Limits

The main objective of this study was to develop an improved method for an application of ductile fracture criteria to predict forming limit diagram (FLD) of the sheet metals. Neck initiation was studied experimentally and numerically for a tensile test. Based on the results, a new methodology regarding the ductile fracture criteria was proposed to estimate forming limits. The new methodology states that the fracture criteria constant could be calculated at a thickness strain in the range of 20–25%, and be considered as a critical value of the ductile fracture criteria for strain localization. The new proposed methodology was successful in predicting the left side of the FLD and more refinement is needed to predict the right side of the FLD.     

Formability evaluation for hot-rolled HB780 steel sheet based on 3-D non-quadratic yield function

A common practice to evaluate formability in the typical sheet metal forming process is to measure hardening behavior and a forming limit diagram as separate material properties, and perform numerical forming simulations utilizing various yield functions. The measured forming limit diagram is applied as the failure criterion. However, the performance of material properties such as hardening behavior and yield functions in predicting strain localization in the simple tension and forming limit diagram tests is seldom validated before their application to forming simulation. In this study, a new numerical formability evaluation procedure was proposed, in which not only hardening behavior but also measured forming limit data were employed in characterizing the input data for the hardening behavior and the yield function. Besides, strain localization was directly monitored to determine failure without employing any forming limit criterion. The new procedure was applied for rather thick advanced high strength hot-rolled steel sheet so that 3-D continuum elements were utilized along with 3-D non-quadratic Hosford and quadratic Hill yield functions.     

基于数值模拟的某轿车后灯座工艺补充面设计

分析了某轿车后灯座的工艺特点,制定了不同的工艺补充面模型,首先采用板料三维成形分析软件DY-NAFORM对其坯料形状进行优化;然后,根据模拟结果,选取两种不同形状的坯料,对该零件的拉深成形工序再次进行数值模拟仿真分析,通过对其FLD图和成形结果的分析、比较,优化了工艺型面,得到了合理的拉深凹模型面,最终生产出合格产品.     

激光拼焊板成形极限的预测方法研究

激光拼焊板成形极限图(FLD)目前主要由试验法获得,但试验法由于其局限性,在实际应用中受到很大的限制。该文介绍了一种预测板料发生颈缩的新准则——厚度梯度准则,该准则基于当板料发生颈缩时沿垂直于颈缩方向的厚度梯度分布上存在着临界值C;在板料成形过程中,当其厚度梯度值小于临界厚度梯度值C时,板料发生颈缩。采用eat/Dynaform软件仿真了高强度钢B170P1激光拼焊板的凸模胀形试验,基于厚度梯度准则有限元计算获得了其成形极限图。获得的激光拼焊板FLD与通过凸模胀形试验法得到的结果进行了对比分析。与试验得到的数据吻合较好,从而证实了该方法的正确性和有效性。     

变压边力对圆筒形件拉伸性能的影响研究

通过利用有限元数值模拟的方法，研究了几种典型的变压边力对于铝合金圆筒形件拉伸成形性能的影响，分析了圆筒形件不同位置板料在成形过程中的应变路径，揭示了铝合金圆筒形件的基本成形原理；研究了铝合金材料在几种典型压边力条件下拉伸成形时的区别，并以板料在出现破裂前所达到的最大减薄率为主要判断依据，对圆筒形件在具有几种典型变化规律的压边力的作用下所能达到的最大拉伸深度进行了研究。结果表明，渐增型和∧型压边力有助于改善铝合金圆筒形件的成形性能。     

Evaluation and prediction of the forming limit of AZ31B magnesium alloy sheets in a cross-shaped cup deep drawing process

In this study, the formability of AZ31B magnesium alloy sheets was investigated through experimental and numerical approaches. Tensile tests and limit dome height tests were carried out at several temperatures between 25 °C and 300 °C to obtain the mechanical properties and forming limit diagram (FLD). The interfacial heat transfer coefficient between two adjacent tools, and the convection coefficient were estimated by comparing the tool temperatures obtained from trial heat transfer analyses with actual measured data. The FLD-based criterion considering the strain path and the blank temperature was used to predict by finite element analysis (FEA) the forming limit in a cross-shaped cup deep drawing process. A comparison of the FEA and experimental data showed that this criterion was very useful and reasonable. In particular, the temperature of each forming tool that provided the best formability of AZ31 sheets was determined by coupled temperature-deformation analyses. A practical method that can greatly reduce the forming time by increasing the punch speed during the forming process was suggested.